The use of satellite positioning systems is becoming increasingly common. Satellite positioning systems are being used in a variety of applications such as navigation (for example automotive navigation) and surveying. Dedicated satellite positioning system receivers are now relatively inexpensive and readily available to the consumer. In addition, satellite positioning system receivers are also now being integrated into mobile telephones, personal digital assistants (“PDA”) and automotive computers, as well as being available as peripherals for personal computers.
An example of a satellite positioning system is the Global Positioning System (“GPS”), operated by the United States Department of Defense. The GPS system uses a satellite constellation of at least 24 satellites in a circular medium earth orbit (“MEO”), such that at least four satellites are within line of sight from almost any point on the Earth, and each satellite orbits the Earth twice every day. Other satellite positioning systems include the Russian Global Navigation System (GLONASS) and the European Galileo system.
A GPS receiver can determine its location in terms of its longitude, latitude and altitude (as well as its speed and the current time) by performing calculations on the signals broadcast from the satellites. The location of the receiver is calculated using a process known as trilateration.
Each satellite continuously transmits a coded identification signal, allowing a receiver to identify the satellite. Along with this signal, the satellite transmits a time signal (from an atomic clock on board the satellite), ephemeris data, and an “almanac” of the location of other satellites. Ephemeris data is a set of information that allows the receiver to calculate the movement of the satellite in orbit over a specific interval of time. Typically, the ephemeris data can enable the receiver to accurately calculate the position of the satellite at any given moment over a period of four hours. The ephemeris data allows a plot to be determined that describes how the satellite orbits the Earth. The plot is in the form of a curve or line which is a good fit for the satellite's orbit for approximately four hours, but for times after this the accuracy of the plot gradually degrades. The almanac is a description of the location of the other satellites in the constellation, but is not as accurate as the ephemeris data, and is used by the receiver to determine which other satellites it can see.
By reading the time signal from the satellites, the receiver can determine the distance from the satellites (known as the “pseudorange”). Therefore, the receiver can determine that its position is on the surface of an imaginary sphere centred on the location of the satellite. With signals from four (or more) satellites, the receiver can determine the intersection point of the four (or more) imaginary spheres, which gives the location of the receiver. If only three satellites are visible to the receiver, a two-dimensional location of the receiver can be calculated.
Ground stations based on the Earth monitor the precise locations of the satellites, and update the ephemeris data on the satellites to match the observations from the Earth. The ground stations also update the atomic clocks on the satellites to synchronise them.
A problem with GPS receivers is that they may typically take about 30 seconds to start up, because the receiver needs to download the ephemeris data broadcast from the satellites, in addition to measuring the timing. As mentioned, once loaded, the ephemeris data is valid for about four hours. However, after about an hour many satellites will have set or risen, so new data is required. This therefore incurs another delay as the new data is downloaded.
In addition, inside buildings or other areas where there is no line of sight to the satellites (such as “urban canyons”), the signal may be too weak to download the ephemeris data. In these situations the location of the receiver cannot be determined. A known solution to this problem is to use a communication link to fetch the ephemeris data from a server based on Earth (called assisted GPS). However, this requires the ability to connect to the server, which may not always be available. For example, if the connection is made using a cellular wireless connection, then there must be sufficient cellular coverage in the area and the receiver must have necessary hardware to connect to a cellular system. Alternatively, if the connection is via the Internet, then the receiver must have access to either a wired or wireless Internet connection, which may be inconvenient to the user and requires extra hardware in the receiver. Furthermore, the downloading of the ephemeris data from a server also incurs delay in determining the location of the receiver.
There is therefore a need for techniques to address the aforementioned problems by, for example, providing ephemeris data to a satellite positioning receiver.